The species of things are distinguished and made by chance, in order to naming and names imposed on those things which either the conveniences of life or common observation bring into discourse. The greatest part of the rest, sine nomine herbæ, lie neglected, neither differenced by names, nor distinguished into species; viz. how many flies and worms are there which, though they are about us in great plenty, we have not yet named nor ranked into species, but come under the general names of flies or worms, which yet are as distinct as a horse and a sheep, though we never have had so great occasion to take notice of them.
So that our ideas of species are almost voluntary, or at least different from the idea of Nature by which she forms and distinguishes them, which in animals she seems to me to keep to with more constancy and exactness than in other bodies and species of things: those being curious engines do perhaps require a greater accurateness for their propagation and continuation of their race; for in vegetables we find that several sorts come from the seeds of one and the same individual, as much different species as those that are allowed to be so by philosophers.
This is very familiar in apples, and perhaps other sorts of fruits, whereof some have distinct names and others only the general, though they begin every day to have more and more given them as they come into use. So that species, in respect of us, are but things ranked into order, because of their agreement inn some ideas which we have made essential in order to our naming them, though what is essentially to belong to any species in reference to Nature be hard to determine; for if a woman should bring forth a creature perfectly of the shape of a man, that never showed any more appearance of reason than a horse, and had no articular language, and another woman should produce another with nothing of the shape, but with the language and reason of a man, I ask which of these you would call by the name man?both or neither?
In questions where there are arguments on both sides, one positive proof is to preponderate to a great many negatives, because a positive proof is always founded upon some real existence, whcih we know and apprehend; whereas the negative arguments terminate generally in nothing, in our not being able to conceive, and so may be nothing but conclusions from our ignorance or incapcity, and not from the truth of things which may, and we have experience do, really exist,
though they exceed our comprehension. This amongst the things we know and lie obvious to our senses is very evident; for though we are very well acquainted with matter, motion, and distance, yet there are many things in them which we can by no means comprehend; for, even in the things most obvious and familiar to us, our understanding is nonplussed, and presently discovers its weakness; whenever it enters upon the consideration of anything that is unlimited, or would penetrate into the modes or manner of being or operation, it presently meets with unconquerable difficulties. Matter, and figure, and motion, and the degrees of both, we have clear notions of; but when we begin to think of the extension or divisibility of the one, or the beginning of either, our understanding sticks and boggles, and knows not which way to turn. We also have no other notion of operation but of matter by motion,at least I must confess I have not, and should be glad to have any one explain to me intelligibly any other; and yet we shall find it hard to make out any phenomenon by those causes. We know very well that we think, and at pleasure move ourselves, and yet, if we will think a negative argument sufficient to build on, we shall have reason to doubt whether we can do one or other; it being to me inconceivable how matter should think, and as incomprehensible how an immaterial thinking thing should be able to move material, or be affected by it. We having therefore positive experience of our thinking and motion, the negative arguments against them, and the impossibility of understanding them, never shake our assent to these truths, which perhaps will prove a considerable rule to determine us in very material questions.